Solderable zinc alloy coating



United States Patent filice SOLDERABLE zlNc ALLOY COATING Edward B. Saubestre, Elmhurst, N.Y., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts No Drawing. Application December 28, 1955 Serial No. 555,782

6 Claims. (Cl. 148-614) The present invention relates to the treatment of metal surfaces to inhibit finger staining and to improve corrosion resistance while retaining a readily solderable surface. In particular, the present invention relates to dips for solderable zinc-tin alloys, and to improved dipping processes for said alloys which provide a solderable, rust resistant coating which is not relatively prone to finger staining and is of attractive appearance.

In application Serial No. 526,860, filed August 8, 1955, in the name of the inventor herein and assigned to the assignee of the present invention, there is described improved plating methods and baths for obtaining a solderable zinc-tin alloy containing between 75 and 95 percent zinc and between 25 and percent tin; This alloy finds many applications in the electronics industry and is an excellent substitute for the scarce and rather expensive cadmium. Solder-able zinc-tin alloys, electrodeposited according to said application, have a white matte finish which is prone to finger staining.

Broadly, it is an object of the present invention to provide an improved dip and dipping process for solderable zinc-tin alloys which provides a finish which is relatively passive to finger staining and which may be handled without danger of adversely altering the appearance of the alloy. Specifically, it is within the contemplation of the invention to provide treatment methods and baths which render electro-deposited zinc-tin alloys less prone to finger staining and white salt formation, Without aifecting the solderability of the alloy.

I have found that the treatment of such electrodeposited zinc-tin alloys with acid oxidizing solutions converts the normally white matte surface of the alloys to an oxidized surface without affecting the important property of solderability. The oxidized surface has a gray finish of attractive appearance which is resistant to corrosion and tarnishing. Known bright dips for zinc electro-deposited coatings, such as those of the nitric acid type, are not effective when foreign metals are present in the zinc bright dip solution; accordingly, known treatment methods for zinc were not expected to find application to the processing of zinc-tin alloys. However, the reverse was found to be true. Further, the desirable surface properties must be broughtabout in a manner consistent with the requirement that the solderability of the zinc-tin alloy be preserved. Thus, conventional chromate conversion coatings cannot be employed since such chromate films interfere with solderability, particularly as the thickness of the chromate coating increases.

Dipping solutions which are suitable for processing articles of zinc-tin alloy clad either by electrodeposition or other means are solutions of strong acids which are oxidizing. Excellent results have been obtained with Patented Apr. 28, 1959 solutions consisting essentially of nitric acid, sulfuric acid and mineral acid-hydrogen peroxide mixtures. The strength of the oxidizing acid solution and the immersion time is selected to convert the normally white, matte surface of the solderable zinc-tin alloy into an oxidized film; such a film, although protective, does not interfere with the solderability of the coating and has a pleasing gray color.

The following specific example is given, without limitation, to more completely explain my invention:

Example.-A base metal clad with solderable zinc-tin alloy having approximately percent zinc and 10 percent tin was immersed in a dip of 1 percent by weight of nitric acid for a period of between 20 seconds to one minute in which time a gray oxidized coating was formed on the exposed surfaces of the article. The dipping process was inhibited by first rinsing in cold water followed by rinsing in hot water and drying. The gray oxidized coating suppressed finger staining while not interfering with solderability. The residues left from the original zinc-tin alloy plating were removed by the dipping process.

A latitude of substitution and modification is intended in the foregoing disclosure; and in some instances some features of the invention will be used without a corresponding use of other features.

What I claim is:

1. A process for treating a solderable zinc-tin alloy containing between 75 and percent zinc and between 25 and 5 percent tin and normally having a white matte surface including the steps of subjecting said surface to the action of an oxidizing acid solution of a strength and for a duration to convert said white matte surface into an oxidized film, and removing said surface from the action of said solution before appreciable buildup of said oxidized film.

2. A process for treating a solderable zinc-tin alloy containing between 75 and 95 percent zinc and between 25 and 5 percent tin and normally having a white matte surface including the steps of subjecting said surface to the action of an oxidizing acid solution selected from the group consisting of nitric acid, sulfuric acid and mineral acid-hydrogen peroxide mixtures and of a strength and for a duration to convert said white matte surface into an oxidized film, and removing said surface from the action of said solution before appreciable building of said oxidized film.

3. An article of an electrodeposited zinc-tin alloy containing 75 to 95 percent zinc and 25 to 5 percent tin and having a thin oxidized film on its surface, said film being gray in color, uniform in thickness and solderable.

4. An article of an electrodeposited zinc-tin alloy containing 75 to 95 percent zinc and 25 to 5 percent tin and having a gray oxidized solderable film of uniform thickness on its surface which is resistant to finger straining.

5. A base metal having a coating of a solderable zinc-tin alloy containing 75 to 95 percent zinc and 25 to 5 percent tin, the surfaces of said coating having an oxidized film of a thickness to preclude finger staining without affecting solderability, said film being gray in color, uniform in thickness and solderable.

6. The method of treating an electrolytically deposited layer of an alloy of zinc and tin, said alloy containing between 75 and 95% zinc and between 25 and 5% tin, the layer having a solderable surface which includes 3 l 5 4 I the step of immersing the surface in a solution of nitric 2,035,380 Wilhelm Mar. 24, 1936 acid so as to form a uniform, gray, oxidized coating 2,154,451 Hull Apr. 18, 1939 over the surface, the coating having the property of 2,154,469 Oplinger Apr. 18, 1939 repressing finger staining and being solderable, remov- 2,276,353 Thompson Mar. 17, 1942 ing the surface from the solution and washing the nitric 5 2,461,228 Miles Feb. 8, 1949 acid solution off from the removed surface. 2,559,812 Watson July 10, 1951 References Cited in the file of this patent B brk G lOTHER SE Z Z & FN S a 1 avam'zing, r e ition, pon UNITED STATES PATENTS 10 Ltd.; 22 Henrietta St., London W0. 2 1950 (pages 122,716 Fobes Jan. 16, 1872 112 and 11 relied 1,923,502 Prier Aug. 22, 1933 Patent No, 2,884,350

April 28, 1959 Edward B. Saubestre It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2,, line 57, for. "straining" read staining (SEAL) Attest:

KARL Ho AXLINE Attesting Oflicer ROBERT C. WATSON Commissioner of Patents 

1. A PROCESS FOR TREATING A SOLDERABLE ZINC-TIN ALLOY CONTAINING BETWEEN 75 AND 95 PERCENT ZINC AND BETWEEN 25 AND 5 PERCENT TIN AND NORMALLY HAVING A WHITE MATTE SURFACE INCLUDING THE STEPS OF SUBJECTING SAID SURFACE AND THE ACTION OF AN OXIDIZING ACID SOLUTION OF A STRENGTH AND FOR A DURATION TO CONVERT SAID WHITE MATTE SURFACE FROM THE AN OXIDIZED FILM, AND REMOVING SAID SURFACE FROM THE ACTION OF SAID SOLUTION BEFORE APPRECIABLE BUILDUP OF SAID OXIDIZED FILM. 